针对超声衍射时差法存在检测精度较差、区域检测可靠性不够和信号信噪比低等问题，提出了一种基于光纤皮秒激光器和高速旋转镜的相控阵激光超声裂纹检测方法。利用有限元方法模拟热弹机制下，建立二维瞬态激光超声力-固耦合模型，产生横波与纵波在缺陷处发生衍射现象，分析了裂纹尖端不同奇异点、声波不同中心频率和相控阵激励源不同位置对声波衍射的影响，通过衍射信号的信噪比和位移幅值两个计算指标来分析变化规律，并进行了实验验证。结果表明：数值模拟与实验结果有较好的一致性，相控阵激光源较传统单束激光源对衍射信号幅值和信噪比有明显的增强作用，纵波衍射信号信噪比较理想；衍射信号幅值随裂纹尖端奇异点增加和声波中心频率减小而增大；信噪比随尖端奇异点增加而增大，随声波中心频率一定范围增加无明显变化，随激光源距离的增加呈现先增加后减小的趋势；缺陷定量分析时计算出的裂纹长度与实际裂纹的误差均不超过6.8%。
Abstract
Aiming at the problems of poor detection accuracy, low reliability of area detection and low signal-to-noise ratio of ultrasonic diffraction time difference method, a phased array laser ultrasonic crack detection method based on fiber picosecond laser and high-speed rotating mirror is proposed. The finite element method is used to simulate the thermoelastic mechanism, and a two-dimensional transient laser ultrasonic thermo-solid coupling model is established to generate S-waves and P-waves to diffract at the defect. The influence of different singularities of crack tip, different center frequency of acoustic wave and different position of phased array excitation source on acoustic diffraction is analyzed and verified by experiment. The results show that the numerical simulation results are in good agreement with the experimental results. Compared with the traditional single beam laser source, the phased array laser source has an obvious enhancement effect on the diffraction signal amplitude and signal-to-noise ratio, and the P-wave diffraction signal is ideal. The amplitude of the diffraction signal increases with the increase of the defect singularities and the decrease of the acoustic wave frequency. The signal-to-noise ratio increases with the increase of the tip singularity, and has no obvious change with the increase of the center frequency in a certain range. With the increase of the laser source distance, the signal-to-noise ratio first increases and then decreases, and the error between quantitative analysis of crack length and actual crack is less than 6.8%.
关键词
衍射时差法 /
相控阵激光 /
有限元 /
裂纹尖端 /
信噪比
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Key words
time of flight diffraction /
phased array laser /
finite element /
crack tip /
signal-to-noise ratio
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